1. Field of the Invention
This invention relates to an electron microscope, and more particularly to an electron microscope in which the brightness of a specimen image is automatically adjusted.
2. Description of the Prior Art
In an electron microscope, the magnification of a specimen image need generally be varied over a wide range. In general, the electron microscope is equipped with the so-called multistage magnifying lens system which comprises three to five magnifying lenses including an objective lens. The magnification can be varied by varying the excitation currents of the magnifying lenses. It is undesirable, however, to vary the excitation currents of the respective magnifying lenses while neglecting an aberration which is caused by changing widely the excitation current of each magnifying lens. For this reason, the excitation currents of the respective magnifying lenses are so pre-determined that the aberration of the magnifying lens system may be minimized at the respective magnifications within the magnification range as actually used, with the result that the highest performance may be attained the whole electron microscope. To manually vary the excitation currents in accordance with the respective magnifications is extremely troublesome, and increases occasions on which erroneous exciting currents are set. Usually, the magnification need not necessarily be varied continuously over the wide range. From these viewpoints, the following measure is taken in the electron microscope. The magnification is varied stepwise by manipulating a magnification setting manipulator (which is generally a knob). The exciting currents of the respective magnifying lenses are set stepwise automatically in interlocking relationship with the magnification setting manipulator to values at which the aberration of the magnifying lens system is minimized at each magnification to be set.
The brightness of a specimen image is inversely proportional to the square of the magnification of the specimen image. Therefore, the specimen image is too bright or too dark in dependence on the magnification of the specimen image.
In general, the brightness of the specimen image is manually adjusted each time the magnification is changed. Such method, however, is troublesome in manipulation and increases occasions of erroneous manipulation. In order to solve the problems, the adjustment of the brightness of the specimen image may be interlocked with the setting of the magnification. In this case, however, there is the problem that a fluctuation in the brightness of the specimen image as attributed to factors other than the magnification, e.g., a fluctuation in an electron beam and light having leaked from the exterior of the electron microscope to the interior thereof, cannot be compensated for.
If, as described above, the brightness must be switched stepwise in interlocking relationship with switching of the magnification stepwise, then the number of wirings in an electric circuit therefor becomes large and the degradation of reliability is inevitable. This problem is more conspicuous as the number of magnifications to be set is larger.